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DDR2 800mhz running at 667mhz?

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February 8, 2007 11:33:06 AM

Hey all - I just built a new system and am using 2 GB of Corsair PC2-6400 DDR2 800MHz. For some reason, my BIOS is defaulting this to run at a DRAM frequency of 667mhz. Do you have any idea why this might be?

I'm not an overclocker, but I am familiar with the BIOS and know how to change this on my motherboard (ASUS P5B Deluxe). However, is it safe to maually change the DRAM frequency to DDR-800 even though the BIOS is defaulting lower?

I just want to make sure that adjusting the memory speed won't negatively impact my computer (Core™ 2 Duo E6600 ) in any way. I'm not looking to overclock, so will bumping up the DRAM frequency make the processor run faster than its rated?

As a first time poster, I really appreciate your help.
February 8, 2007 11:59:37 AM

Quote:
.... However, is it safe to maually change the DRAM frequency to DDR-800 even though the BIOS is defaulting lower?

....


yes.
February 8, 2007 12:05:00 PM

Quote:
Hey all - I just built a new system and am using 2 GB of Corsair PC2-6400 DDR2 800MHz. For some reason, my BIOS is defaulting this to run at a DRAM frequency of 667mhz. Do you have any idea why this might be?

Yes. Your DDR2 frequency is now at 667MHz (bus=333MHz.)
That freq is probably set by the SPD.

Quote:
However, is it safe to maually change the DRAM frequency to DDR-800 even though the BIOS is defaulting lower?

Yes, it is safe.

Quote:
I just want to make sure that adjusting the memory speed won't negatively impact my computer (Core™ 2 Duo E6600 ) in any way. I'm not looking to overclock, so will bumping up the DRAM frequency make the processor run faster than its rated?

Probably yes since you must set the bus speed to 400Mhz to achieve DDR2 freq 800MHz.

Quote:
As a first time poster, I really appreciate your help.

Welcome. Hope it helps.
Related resources
February 8, 2007 12:07:26 PM

Quote:
Quote:

I just want to make sure that adjusting the memory speed won't negatively impact my computer (Core™ 2 Duo E6600 ) in any way. I'm not looking to overclock, so will bumping up the DRAM frequency make the processor run faster than its rated?

Probably yes since you must set the bus speed to 400Mhz to achieve DDR2 freq 800MHz.

Quote:
As a first time poster, I really appreciate your help.

Welcome. Hope it helps.

Only if the memory multiplier is 2. If the memory multiplier us 3, the bus can ruin at 266MHz. At 2.5 its 320
February 8, 2007 12:11:56 PM

Quote:
Hey all - I just built a new system and am using 2 GB of Corsair PC2-6400 DDR2 800MHz. For some reason, my BIOS is defaulting this to run at a DRAM frequency of 667mhz. Do you have any idea why this might be?

I'm not an overclocker, but I am familiar with the BIOS and know how to change this on my motherboard (ASUS P5B Deluxe). However, is it safe to maually change the DRAM frequency to DDR-800 even though the BIOS is defaulting lower?

I just want to make sure that adjusting the memory speed won't negatively impact my computer (Core™ 2 Duo E6600 ) in any way. I'm not looking to overclock, so will bumping up the DRAM frequency make the processor run faster than its rated?

As a first time poster, I really appreciate your help.


As somebody else mentioned, the SPD chip on your RAM is probably reporting 667 MHz thought the RAM itself is tested and rated for 800 MHz. All that you need to do is to change the setting manualy in the BIOS.

Changing the FSB to 400 MHz is not the same thing. Both the CPU and RAM run off the FSB so you would be overclocking your CPU at that point while the RAM would be right-clocked. Your CPU can probably take the 400 MHz FSB. If overclocking interests you, there are lots of threads and stickys in this forum that will guide you.
February 8, 2007 12:11:56 PM

Wow, I love the quick replies on here! I will go ahead and manually change the DRAM to run at 800mhz, but like sruane mentioned, I will essentially be overclocking since the bus speed needs to be bumped to 400Mhz? I'm not overly concerned since this setup is a known to be solid at handling such demands.

As much as I've researched, I don't fully understand how the CPU FSB & RAM frequencies are connected. I know it has something to do with ratios, etc. Is there a good guide for newbies like myself that can explain this a little better?
February 8, 2007 12:15:11 PM

Quote:
Wow, I love the quick replies on here! I will go ahead and manually change the DRAM to run at 800mhz, but like sruane mentioned, I will essentially be overclocking since the bus speed needs to be bumped to 400Mhz? I'm not overly concerned since this setup is a known to be solid at handling such demands.


The bus speed does not need to be raised to 400MHz, in fact - I'd advise against it. Running the bus that fast will require that you make several other tweaks. Your best bet is to just change the memory multiplier.
February 8, 2007 12:25:04 PM

Quote:
Wow, I love the quick replies on here! I will go ahead and manually change the DRAM to run at 800mhz, but like sruane mentioned, I will essentially be overclocking since the bus speed needs to be bumped to 400Mhz? I'm not overly concerned since this setup is a known to be solid at handling such demands.


The bus speed does not need to be raised to 400MHz, in fact - I'd advise against it. Running the bus that fast will require that you make several other tweaks. Your best bet is to just change the memory multiplier.

Will changing the memory multiplier alter the CPU in any way? I'm figuring ASUS is defaulting the RAM to 667 for a reason.
February 8, 2007 12:30:57 PM

Quote:
Wow, I love the quick replies on here! I will go ahead and manually change the DRAM to run at 800mhz, but like sruane mentioned, I will essentially be overclocking since the bus speed needs to be bumped to 400Mhz? I'm not overly concerned since this setup is a known to be solid at handling such demands.


The bus speed does not need to be raised to 400MHz, in fact - I'd advise against it. Running the bus that fast will require that you make several other tweaks. Your best bet is to just change the memory multiplier.

Will changing the memory multiplier alter the CPU in any way? I'm figuring ASUS is defaulting the RAM to 667 for a reason.

No.
February 8, 2007 12:53:48 PM

How do I go about changing just the memory multiplier in the BIOS? I assume I have to keep SPD disabled (which I did anyway to correct the CAS ratings). Ideally, I want to leave the board FSB's alone and just have the RAM run at the speed it's rated for. Thanks for your help.
February 8, 2007 1:38:59 PM

FWIW, if your are not overclocking and your RAM is running at 667MHz then your memory multiplier has already been changed from 1:1 to something like 4:5 CPU:D RAM.

Have you looked at what CPU-Z reports for your system?? In particular, I'd be curious what CPU-Z reports about the DRAM's speed and SPD settings. Not sure why everyone else is speculating about what your SPD does or does not contain when you can just run CPU-Z and find out ... :? If this is DDR2-800 RAM then I would expect there would be SPD settings for 400MHz (as well as 333 and 266).

Whatever. Why not just look and see? :) 

-john, the ostensibly clueless redundant legacy dinosaur
February 8, 2007 2:14:30 PM

Quote:
FWIW, if your are not overclocking and your RAM is running at 667MHz then your memory multiplier has already been changed from 1:1 to something like 4:5 CPU:D RAM.

Have you looked at what CPU-Z reports for your system?? In particular, I'd be curious what CPU-Z reports about the DRAM's speed and SPD settings. Not sure why everyone else is speculating about what your SPD does or does not contain when you can just run CPU-Z and find out ... :? If this is DDR2-800 RAM then I would expect there would be SPD settings for 400MHz (as well as 333 and 266).

Whatever. Why not just look and see? :) 

-john, the ostensibly clueless redundant legacy dinosaur


Thanks, John - I'll take a look and see. In the meantime, if I change my memory multipiler to 3, the FSB will still run at 266Mhz (stock speed ), correct? My goal is to keep the system at 2.4Ghz, 266Mhz FSB, with the DRAM at 800Mhz. Would this be a 1:1 ratio?
February 8, 2007 2:39:22 PM

Quote:
FWIW, if your are not overclocking and your RAM is running at 667MHz then your memory multiplier has already been changed from 1:1 to something like 4:5 CPU:D RAM.

Have you looked at what CPU-Z reports for your system?? In particular, I'd be curious what CPU-Z reports about the DRAM's speed and SPD settings. Not sure why everyone else is speculating about what your SPD does or does not contain when you can just run CPU-Z and find out ... :? If this is DDR2-800 RAM then I would expect there would be SPD settings for 400MHz (as well as 333 and 266).

Whatever. Why not just look and see? :) 

-john, the ostensibly clueless redundant legacy dinosaur


Thanks, John - I'll take a look and see. In the meantime, if I change my memory multipiler to 3, the FSB will still run at 266Mhz (stock speed ), correct? My goal is to keep the system at 2.4Ghz, 266Mhz FSB, with the DRAM at 800Mhz. Would this be a 1:1 ratio?

That is correct. I have the same memory as you do and i am running it at a 1:1 multiplyer and thats because i have my system overclocked. But since your not overclocking keep it on the multiper 3 ratio.
February 8, 2007 2:46:41 PM

Quote:
FWIW, if your are not overclocking and your RAM is running at 667MHz then your memory multiplier has already been changed from 1:1 to something like 4:5 CPU:D RAM.

Have you looked at what CPU-Z reports for your system?? In particular, I'd be curious what CPU-Z reports about the DRAM's speed and SPD settings. Not sure why everyone else is speculating about what your SPD does or does not contain when you can just run CPU-Z and find out ... :? If this is DDR2-800 RAM then I would expect there would be SPD settings for 400MHz (as well as 333 and 266).

Whatever. Why not just look and see? :) 

-john, the ostensibly clueless redundant legacy dinosaur


Thanks, John - I'll take a look and see. In the meantime, if I change my memory multipiler to 3, the FSB will still run at 266Mhz (stock speed ), correct? My goal is to keep the system at 2.4Ghz, 266Mhz FSB, with the DRAM at 800Mhz. Would this be a 1:1 ratio?

That is correct. I have the same memory as you do and i am running it at a 1:1 multiplyer and thats because i have my system overclocked. But since your not overclocking keep it on the multiper 3 ratio.

Thanks, xeni. What does the memory multiplier essentially do? Is it simply telling only the RAM to operate at a different speed relative to the FSB (i.e., 2 x 266, 3 x 266, etc.)?
February 8, 2007 2:57:37 PM

I have the same issue. Well, not really issue cause I know how to fix it but I have a question.

I've heard it's more stable to run your CPU:D RAM ratio at 1:1 than some other weird ratio. My E6600 is running at 3.0 Ghz, 333 Mhz bus speed and so my memory is running at DDR2-667. Would it be better to leave it as it is now or change the ratio and run my memory at DDR2-800 like it's rated for?
February 8, 2007 3:37:36 PM

Quote:
I have the same issue. Well, not really issue cause I know how to fix it but I have a question.

I've heard it's more stable to run your CPU:D RAM ratio at 1:1 than some other weird ratio. My E6600 is running at 3.0 Ghz, 333 Mhz bus speed and so my memory is running at DDR2-667. Would it be better to leave it as it is now or change the ratio and run my memory at DDR2-800 like it's rated for?


From what I've read, 1:1 is the way to go, but it seems that involves at least some degree of overclocking. The other posters can correct me if I'm wrong, but the Core 2 Duo's seem to have a negligible performance increase in memory speeds above 533Mhz. I probably could keep mine at 667, but I kinda feel like I'm not getting what I paid for if it's not running at full capacity.
February 8, 2007 3:46:48 PM

Quote:
In the meantime, if I change my memory multipiler to 3, the FSB will still run at 266Mhz (stock speed ), correct? My goal is to keep the system at 2.4Ghz, 266Mhz FSB, with the DRAM at 800Mhz. Would this be a 1:1 ratio?

My arithmetic is different. I get 400MHz = 1.5*266MHz or a memory multiplier of 1.5. I don't think the memory multiplier is typically specified this easily though. Usually the motherboard manufacturer will try to "help" by either allow you to pick from a list of CPU:D RAM ratios (266:400 or 2:3 being what you'd want in this case) or, as Asus appears to do, giving you a list of memory freqs to pick from.

Info about changing the memory multiplier should be coved in the Asus P5B Deluxe owner's manal. (If you can't find yours you can download it from Asus. Here's a link to the Asus download page). I just glanced through the manual quickly so I'm not positive, but I think that the memory multiplier is set under "JumperFree Configuration". (Page 87 in my downloaded copy). Here is what I think is the path to this setting in the BIOS: Main->Advanced->JumperFree Configuration ... DRAM Frequency

The default setting is "Auto". Other choices appear to be DDR2-533, DDR2-667, DDR2-800, DDR2-899, DDR2-1067. All of these of course assume that you have set the CPU Frequency to either Auto or 266 (i.e. no overclocking).

-john, the ostensibly clueless redundant legacy dinosaur
February 8, 2007 4:00:07 PM

Quote:
Not sure why everyone else is speculating about what your SPD does or does not contain when you can just run CPU-Z and find out ... :?


Some DDR2 modules only have SPD settings to 333 MHz even though they are rated for 400 MHz operation. The Kingston Hyper-X memory that I use is an example of that. CPU-Z reports that it is rated for 333 MHz maximum even though it is tested, marketed and sold as 800 MHz memory.
February 8, 2007 4:54:33 PM

Quote:
...
I've heard it's more stable to run your CPU:D RAM ratio at 1:1 than some other weird ratio.

Nope; weird ratios are stable, too.

Quote:
y E6600 is running at 3.0 Ghz, 333 Mhz bus speed and so my memory is running at DDR2-667. Would it be better to leave it as it is now or change the ratio and run my memory at DDR2-800 like it's rated for?
The main thing is to be running in dual-channel mode at at least a 1:1 ratio. If you speed up the memory one notch (as you suggest), it probably won't make a big performance difference. Speeding up the memory two or more notches will make a more substantial difference.
February 8, 2007 8:23:29 PM

Quote:
I've heard it's more stable to run your CPU:D RAM ratio at 1:1 than some other weird ratio.

Nope; weird ratios are stable, too.
Yes, weird ratios are stable in the sense that you shouldn't crash or BSOD your system. But there is also apparently some performance weirdness. :) 

Not quite a week ago in another thread someone else pointed me towards this Madshrimps article: Intel Core 2: Is high speed memory worth its price? Basically there take an E6700, 2GB of fast OCZ DDR2-900 PC7200, and an Intel D975XBX motherboard and see what happens running the same benchmarks with the CPU at stock 266MHz speed while varying the memory speed and timing settings in the BIOS.

What's surprising is that they don't always see a performance increase when one would naively expect one. For example, when they compare DDR2-533(4-4-4-12) to DDR2-667(5-5-5-15) the total latencies should be approximately the same. And if the latencies are the same, I would expect the higher freq DDR2-667 to perform slightly better than the DDR2-533. However, they actually measure a slight drop in the performance results for their Super-Pi and the Photoshop benchmarks.

In other words, something a little funky seems to happen when running the DRAM one "notch" (?) above the FSB speed.

Don't know what, if anything, to conclude from this article. It's probably important to remember that whatever differences they did notice were all very small. No one is likely to notice a performance difference on the order of 5% or less other than by measuring it. Human perceptions are just not that finely honed.

I also wonder what would happen if this test were run against an Intel P965 chipset motherboard instead of the P975 chipset. Probably not a significant factor, but I still wonder.

-john, the ostensibly clueless redundant legacy dinosaur.
February 8, 2007 9:02:48 PM

The last paragraph of the Madshrimps article zjohnr refers to does say that 667 memory may actually not be a benefit if not overclocking, but it goes on to say:


Quote:
Why is PC6400, which is also running asynchronous, faster then? Because the memory speed is now that much higher that it compensates for the loss of running asynchronous and overall performance does increase .


http://www.madshrimps.be/?action=getarticle&number=4&ar...
February 9, 2007 12:14:40 PM

A few articles I've read strongly discourage increasing the FSB to 400 on this board. If I want to keep the FSB at 266, is my only option to run at a ratio other than 1:1? And is that really a bad thing? Obviously 1:1 will yield the best performance on benchmarks, but would the normal user notice any difference?
February 9, 2007 5:04:54 PM

Quote:
A few articles I've read strongly discourage increasing the FSB to 400 on this board. If I want to keep the FSB at 266, is my only option to run at a ratio other than 1:1? And is that really a bad thing?

Well, if you are not overclocking then you will always be a long way away from a 400MHz FSB. The FSB and the memory bus are two different things. They are interconnected only in that the speed the memory bus is run at is derived from the FSB speed. Hence the "ratios".

If you want to run the FSB at the stock setting of 266MHz then, yes, the only way to run the memory faster than 266MHz (which is DDR2-533) is to use a ratio other than 1:1.

But no, you are not likely to ever notice the performance difference between any of these settings. We're talking differences of a few percent here. If you had an application that took an hour of CPU and memory intensive crunching to complete, a 5% performance improvement would allow it to complete 3 minutes faster or in 57 rather than 60 minutes. We're talking about performance differences less than 5% so the gain would actually be less.

People tend to spend a lot of money for high performance memory. But the money spent is usually not worth the performance they actually gain. An exception to this would be if they overclocked. But the performance gain in that case is not from the faster memory but because the processor is running faster.

Like I said earlier, the best thing to do is probably to run CPU-Z and see what it says about how fast your CPU and memory are running and also what SPD settings your memory comes with. When you get a chance to do that post the numbers and we can discuss this further if you wish.

-john, the ostensibly clueless rudundant legacy dinosaur
February 9, 2007 5:43:36 PM

Quote:
People tend to spend a lot of money for high performance memory. But the money spent is usually not worth the performance they actually gain. An exception to this would be if they overclocked. But the performance gain in that case is not from the faster memory but because the processor is running faster.


That is true. Expensive memory is important for OC'ers because it introduces more flexibility abling to reach higher CPU speeds. Especially if you have a mobo that allows only changing the bus speed (fixed cpu and dram multipliers) or you have a mobo that allows you to reach higher mem speeds.
The gain is in the CPU OC and bus speed increase, and not on the memory only by itself.

For a common user or a light-OC'ers, DDR2-533 or DDR2-667 is more than enough and what really matters is that you either install 1GB or 2GB capacity (or more!).
February 11, 2007 3:50:15 AM

Quote:
If you want to run the FSB at the stock setting of 266MHz then, yes, the only way to run the memory faster than 266MHz (which is DDR2-533) is to use a ratio other than 1:1.

Sorry about the hijack but I thought I had this figured out and now am confused again (all I'm trying to figure out is the fastest/most economical 2 x 1G RAM for msi P965 mb that will never be oc).

The msi site says "p965 fsb 533, 800, 1066" - does this mean I get to pick between those 3 speeds for fsb? But I'm confused because if ppl are saying 'fsb 400 is pushing it' how come the lowest fsb on p965 is substantially higher than 400?

I'm also confused (again) because:

1) should I be getting DDR2-533 because this is 1:1 what the lowest p965 fsb can be set to (533MHz) or because

2) it's not really fsb 533MHz but 266MHz and DDR2-533 is 2x that or because

3) DDR2-533 is half the highest fsb can be set to (1066MHz)?

Really, all I'm after is trying to determine the fastest, least expensive and most reliable RAM I can get for this board but after all this reading I know less than when I started :) 

Thank you.
February 11, 2007 7:00:25 PM

mark,
The FSB *clock* is 266MHz for Core2Duo CPUs. However, Intel CPUs transfer 4 pieces of data per clock cycle, so the data rate is 1066MHz. Most recent Pentium 4/D CPUs run at a 200MHz (=800MHz data rate) FSB. Older Pentium 4/D CPUs run at a 133MHz (=533MHz data rate) FSB.

For memory, the key is that you want the memory throughput to be at least as high as the FSB's data rate, so the FSB will not have to "wait" for the memory bus.
Thus, if you are using a non-OC'd C2D CPU, the FSB data rate is 1066MHz, so we want to match our memory throughput to that for a 1:1 ratio. DDR2-533 has a 533MHz data rate (but 266MHz clock speed), and that data rate gets doubled when the system is running in dual-channel mode; 533Mhz data rate x 2 (dual channel mode) = 1066MHz data rate, matching the non-OC'd C2D CPU FSB data rate.

WRT your post, the msi site is talking about FSB data rate, not clock rate. However, most of the FSB speed references in this article are to clock rate, not data rate.

Bottom line, for people who will never ever never overclock, get DDR2-533. But since it seems pretty silly not to overclock C2D CPUs at least a bit, others should get DDR2-667 or DDR2-800.
February 11, 2007 7:05:52 PM

I sometimes think that terminology used for CPU, the FSB, and DRAM speeds has been deliberately designed (by Satan?) to confuse and mislead us all. More likely what happened is that each time the technology was extended the terminology was modified in a way that seemed reasonable at the time. But after a few rounds of this, the end result is that the naming conventions appear to not make any sense ... unless you know the underlying context.

I'll try to describe how I think things work. Hopefully someone else will correct me if I say something that is not actually true. I'll start with the flow diagram below of one of Intel's older chipsets. At the top of the diagram is the CPU (a Pentium 4 in this case). The line labeled "6.4GB/s" represents the FSB (Front Side Bus). The FSB connects the CPU to the Memory Controller Hub (MCH). (The MCH is also referred to as the "northbridge"). To the right of the MCH you see two lines going from the MCH to DDR2. This represents the two independent memory bus connections from the MCH to the memory available when dual channel memory access is enabled.



Perhaps a good place to start is to point out that the FSB is not connected directly to anything other than the MCH. In particular, the FSB does not talk directly to the memory. Only the MCH can access the memory and it does so on a separate interface. This is why it is possible to run the memory and the FSB at different speeds.

Now about the different speeds ... Where to begin?? Let's start with what I call the "system clock". This is the underlying clock which all the other clocks are derived from through multipliers, ratios, dividers, or whatever. :roll: There are a few standard basic ("stock") clock speeds that CPUs have used recently. They are 133MHz, 200MHz, 266MHz, and (just now being introduced) 333MHz.

However, the speeds usually quoted for the FSB are derived by multiplying the system clock by 4. The rational behind this is that the FSB is "quad-pumped". This is marketing lingo meaning the FSB can move 4 "units" of data every time the underlying system clock cycles. So, multiplying by 4, we have effective FSB speeds of 533, 800, 1066, and 1333. These should all look familiar to you, no?

When you see a comment like 'FSB 400 is pushing it', translate it to 'FSB 1600 is pushing it' and it should then make more sense relative to the specs provided for motherboards. This dual usage is actually pretty common. I'll often refer to FSB 266 when what I suppose I should technically say is FSB 1066. The base system clock speeds just seem more real to me. So I assume/hope that the implied "factor of 4" will be "obvious" to most.

It's also easier for me to relate a system clock speed to a DDR2 effective speed since the DDR2-xxx speed is just 2*(system clock) when using a 1:1 ratio. So for example, assuming a 1:1 CPU:D RAM ratio and a stock clock speed of 266MHz we get DDR2-533 and FSB 1066. Finally, if dual channel memory support is enabled then there are actually two DDR2-533 memory DIMMs which the MCH can access simultaneously. The effective memory access speed in that case is 2*533 = 1066. The same effective speed as the FSB. Wow. :) 

OK, I've droned on to the point where I'm not sure I'm helping any longer so I'll stop here. If you still have questions, post them and I or someone else will take another crack at answering them. :wink:

Hope this helped at least a bit ...

-john, the ostensibly clueless redundant legacy dinosaur
February 11, 2007 7:10:40 PM

Nice post zj! My one quibble is that I've typically seen "B" used for bytes and "b" used for bits, so MB/s would be megabytes/sec, and Mb/s would be megabits/sec.
February 11, 2007 7:16:55 PM

Quote:
Nice post zj! My one quibble is that I've typically seen "B" used for bytes and "b" used for bits, so MB/s would be megabytes/sec, and Mb/s would be megabits/sec.

Yeah, you're right. After thinking about it I decided I didn't really understand the figures in the chart yet so I deleted my "P.S." from the post. I understand the memory bandwidth (I think), but I'm not quite getting how an FSB of 800 (I'm guessing) translates into 6.4 giga-bytes/sec ... :?

Arithmetic! :evil: 

Edit: Well, reflecting on it, I guess Intel must use at least a 64-bit or 8-byte wide bus for the FSB so 800MT/s would move 8*800MB/s or 6.4GB/s. I obviously get "fuzzy" when talking about the FSB. :oops: 

Part of the problem is that I've never seen an explanation for how the so-called "quad-pumping" is implemented. Perhaps the bus is actually 16-bytes wide and run dual-data rate the same as the memory bus? I just don't know. None of the descriptions I've seen ever goes further than that "magic" multiplier of 4. :?

Edit: Hmmmmm. According to the entry in the "Host Interface Signals" table on page 38 of the spec for the 965P chipset, the FSB only has 64 data lines (HD[63:0]) so it's only 8 bytes wide. How on earth they manage to move data on a 8 byte wide FSB 4 times per clock cycle is ... a question which I don't know the answer to. :) 

-john
February 11, 2007 9:12:44 PM

Thank you both for the help (good reading too); mondoman, your first reply to me got me straightened out - really appreciate it.
February 11, 2007 10:04:00 PM

Quote:
(...)There are a few standard basic ("stock") clock speeds that CPUs have used recently. They are 133MHz, 200MHz, 266MHz, and (just now being introduced) 333MHz.

(...) So, multiplying by 4, we have effective FSB speeds of 533, 800, 1066, and 1333. These should all look familiar to you, no?

(...) So for example, assuming a 1:1 CPU:D RAM ratio and a stock clock speed of 266MHz we get DDR2-533 and FSB 1066. Finally, if dual channel memory support is enabled then there are actually two DDR2-533 memory DIMMs which the MCH can access simultaneously. The effective memory access speed in that case is 2*533 = 1066. The same effective speed as the FSB.


Great post.

Example and some more questions:
E4300 (FSB=800MHz)
System bus = 200MHz
If i install a dual channel kit DDR2-800 than this means:
2x(2x200MHz) per channel downsized by the MCH?
If i install a dual channel kit DDR2-533 does this means:
2x(4/3 x200MHz) per channel downsized by the MCH?

Without dual channel in this case a single DDR2-533 dimm probably leads to CPU periodically waiting for more data from memory?

Since the MCH has a double simultaneous access to both channels, does this means that in theory a dual channel kit DDR2-800 can reach effective memory bus speeds of 1600MHz?

This means nowadays no matter what cpu you install you can also install DDR2 memory modules with any speeds (533, 667, 800, ...) since the memory bus is absolutely independent from the FSB. Is this also correct?

Thanks a lot.
February 11, 2007 11:59:20 PM

Quote:
...
If i install a dual channel kit DDR2-800 than this means:
2x(2x200MHz) per channel downsized by the MCH?

I don't really understand what you mean by "downsized". DDR2-800 running in dual-channel mode will have a bandwidth of 800MHz x 2 = 1600MHz data rate.

Quote:
...Without dual channel in this case a single DDR2-533 dimm probably leads to CPU periodically waiting for more data from memory?
Yes.

Quote:
...Since the MCH has a double simultaneous access to both channels, does this means that in theory a dual channel kit DDR2-800 can reach effective memory bus speeds of 1600MHz?

Yes. Just to clarify, "dual channel mode" is determined by the motherboard hardware and has nothing to do with the specific memory module. Earlier chipsets/memory controllers were more sensitive to differences in the memory modules in the two channels; "dual channel kits" came about as a convenient marketing solution for this - they are simply two ordinary memory modules from the same production run packaged together.

Quote:
...This means nowadays no matter what cpu you install you can also install DDR2 memory modules with any speeds (533, 667, 800, ...)
Yes.

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... since the memory bus is absolutely independent from the FSB.
In practice, mostly yes. The degree of independence depends on the motherboard design. Underneath it all, the memory bus clock is some integer ratio of the system clock. However, mostly in order to facilitate overclocking, MB makers have added circuitry and BIOS code that allows close to 1MHz control of memory bus speed by increasing the choice of integer ratios. On my NI8 motherboard, this becomes clear in the BIOS in the memory bus speed selection entry. You can manually set the desired speed in 1MHz increments, but the BIOS will automatically "round" that speed to the nearest speed supported by the available ratios. Normally, this only results in a 4-8MHz difference.
February 12, 2007 12:19:31 AM

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Example and some more questions:
E4300 (FSB=800MHz)
System bus = 200MHz
If i install a dual channel kit DDR2-800 than this means:
2x(2x200MHz) per channel downsized by the MCH?
If i install a dual channel kit DDR2-533 does this means:
2x(4/3 x200MHz) per channel downsized by the MCH?

Not quite. The xxx speed in the DDR2-xxx convention is always 2*(memory bus speed). If you are using a 1:1 ratio, then the memory bus is running at the same speed as the system clock. In your example the system clock is 200MHz and I'm assuming the ratio is 1:1 so what is required is DDR2-400 (2*200MHz), not DDR2-800.

The rule with memory is that you can always run it slower than it is rated. So in this case, you could install either DDR2-800 or DDR2-533 DRAM and then run it at DDR2-400 speeds. If you wanted to run the memory at its rated speed you'd first need to use a motherboard that supported this. Assuming the motherboard supports it, you'd use a ratio of 200:266 or 3:4 for DDR2-533 and 200:400 or 1:2 for DDR2-800.

(Of course, almost any recent motherboard would support this. I just don't want to give the impression that all motherboards will let you run any memory at any speed you want. It depends on what the motherboard's BIOS and chipset will support).

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Without dual channel in this case a single DDR2-533 dimm probably leads to CPU periodically waiting for more data from memory?

That's my understanding of it. This is why running your memory in dual channel mode is so strongly recommended. There is typically a significant performance increase (10-15%?? IIRC) using dual channel versus single channel.

To throw numbers at it ... The FSB is 8 bytes wide so running the FSB at 800 MT/s implies a theoretical bandwidth of 6.4GB/s. The max theoretical bandwidth for single channel DDR2-533 is 8 bytes * 533 MT/s or ~4.2GB/s. The CPU and FSB could potentially process data faster than the memory which would lead to the memory limiting the system's performance.

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Since the MCH has a double simultaneous access to both channels, does this means that in theory a dual channel kit DDR2-800 can reach effective memory bus speeds of 1600MHz?

I think I see where you're going and the math works out OK but this is not usually the way it is described. Usually what is stated is the theoretical maximum peak bandwidth. A single DDR2-800 DIMM operating at its rated speed can ... ignoring latencies ... theoretically delivers 8 bytes 800 million times a second. So the single channel peak bandwidth is 8 Bytes*800MT/s or 6.4GB/s. Dual channel theoretically doubles the bandwidth to 12.8GB/s.

In real life you can't ignore latencies (and other limitations) so you are always well below the theoretical limits. Companies do like to use the theoretical limits in their marketing materials though. :) 

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This means nowadays no matter what cpu you install you can also install DDR2 memory modules with any speeds (533, 667, 800, ...) since the memory bus is absolutely independent from the FSB. Is this also correct?

Close ... and probably true enough for the subset of CPUs and memory you are likely to consider buying. However, remember that the speed you can run your memory at relative to the FSB is always determined primarily by what your motherboard's BIOS and chipset will support.

I think that saying the memory bus is absolutely independent of the FSB is wrong because it goes too far. The memory bus is a separate bus and it can be run at a different clock speed. However, that speed is derived from the clock the FSB uses. The FSB and memory bus while separate objects are still inter-dependent.

There may also be other limitations. For example, if you look on page 12 of Intel's overview for the 965 chipset , you'll find a table of Valid FSB/Memory Speed Configurations. The only thing unusual in the table is for FSB 533 (aka system clock 133MHz). When the FSB is 533 apparently the chipset won't allow running the DRAM faster than DDR2-533 speeds. A note below the table states that the 965 chipset "does not support system memory frequencies that exceed the frequency of the Front Side Bus".

It is very unlikely that anyone will run a motherboard with the 965P chipset at FSB 533 so this restriction is not an important one. I'm just dragging it out to illustrate how design dependencies are another way the FSB and memory bus can be inter-dependent. :oops: 

-john, the ostensibly clueless redundant legacy dinosaur
February 12, 2007 8:55:10 AM

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...
If i install a dual channel kit DDR2-800 than this means:
2x(2x200MHz) per channel downsized by the MCH?


I don't really understand what you mean by "downsized". DDR2-800 running in dual-channel mode will have a bandwidth of 800MHz x 2 = 1600MHz data rate.

I was thinking that, in the E4300 example, that MCH would limit the effective speed between FSB and RAM (assuming that if FSB=800MHz than the dual channel speed would be downsized/reduced to 2x400MHz...) in a 1:1 ratio. I guess this was a wrong idea since those bus are independent.

Thanks for the great explanation!
February 12, 2007 9:09:03 AM

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The rule with memory is that you can always run it slower than it is rated. So in this case, you could install either DDR2-800 or DDR2-533 DRAM and then run it at DDR2-400 speeds. If you wanted to run the memory at its rated speed you'd first need to use a motherboard that supported this. Assuming the motherboard supports it, you'd use a ratio of 200:266 or 3:4 for DDR2-533 and 200:400 or 1:2 for DDR2-800.


Assume now i have a motherboard that allows me to set those ratios manually if i want to (and for the record it has a E4300 installed on it).
If i do _not_ set the ratios manually - keep at stock speeds and auto config - what usually does the motherboard configures in this case if i install a dual channel kit DDR2-800? Does it configs at maximum speed ratio 1:2 (2x 800MHz) or does it keeps 1:1 ratio and configs it at 2x 400MHz? (i assume the second choice. Is it correct?)

What i'm getting aware is that a dual-channel kit DDR2-533 with low latency is more than enough to support a E6600 without OC and a dual-channel kit DDR2-667 is more than enough to allow a light/medium OC (say about more 30% on system clock and performance).

Thanks a lot for the great explanation!
February 12, 2007 3:18:32 PM

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I was thinking that, in the E4300 example, that MCH would limit the effective speed between FSB and RAM ...

It's complicated and I don't really know exactly how things work here. But speaking in vague generalities, the speed of the FSB/DRAM system will be limited by the bus with the smallest available bandwidth. If the FSB runs at an effective speed of 800MT/s and has a bandwidth of 6.4GB/s then it doesn't make sense for your memory subsystem to have a bandwidth far greater than this. It won't hurt if the memory can move data much faster than the FSB, but it won't help much either. This is why the earlier Intel motherboards nominally only supported DDR2-533. Running the memory faster wasn't expected to make any difference. When the effective FSB speed was 533MT/s (133MHz) or 800MT/s (200MHz), then a dual channel enabled theoretical memory bandwidth of ~8.5GB/s was good enough.

Currently with DDR2-800 and dual channel the theoretical peak bandwidth of the memory is ~(8*800MT/s)*2 or ~12.8GB/s. That is at least one of the reasons why Intel keeps increasing the speed of their FSB. For all the current Core 2 processors other than the E4300 the current FSB effective speed is 1066MT/s and the theoretical bandwidth is ~8.5GB/s. In a few months (or less) Intel is expected to begin selling chipsets and Core 2 processors which use a 1333MT/s FSB with a bandwidth of ~10.7GB/s. The bandwidth of the FSB and the memory will then be on a more equal footing.

But as I said, it's complicated. For one thing, the peak bandwidth is only important to the extent that the CPU is actually limited by the available FSB bandwidth. Usually it isn't. For another, the CPU (using the FSB) is not the only subsystem on the motherboard which accesses memory. If you look back at that diagram I posted earlier, you'll notice that in addition to the FSB there are also connections to the MCH for the PCI Express graphics and for the "Southbridge". These systems can also access the memory independently of the processor. So, again "theoretically", any extra memory bandwidth is not necessarily wasted because it can be used to satisfy memory read/write requests which originate from sources other than the FSB.

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If i do _not_ set the ratios manually - keep at stock speeds and auto config - what usually does the motherboard configures in this case if i install a dual channel kit DDR2-800? Does it configs at maximum speed ratio 1:2 (2x 800MHz) or does it keeps 1:1 ratio and configs it at 2x 400MHz?

I honestly don't know. I think the latter case ... the 1:1 ratio ... is the most likely, but I am only speculating. Best thing to do is to go into the BIOS and see what settings it picks. You can also use a tool along the lines of CPU-Z or PC Wizard to see how fast things are running.

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What i'm getting aware is that a dual-channel kit DDR2-533 with low latency is more than enough to support a E6600 without OC and a dual-channel kit DDR2-667 is more than enough to allow a light/medium OC (say about more 30% on system clock and performance).

Yes. If you are not overclocking ... an important disclaimer around here since so many people do overclock :)  ... then increasing the memory speed in a Core 2 based system does not increase performance all that much. The performance gained is not going to be worth the extra money spent for the faster memory. Premium memory only makes sense if you're going to overclock ... or if you don't pay extra for it, I suppose. :) 

I should probably point out that while the above is true for the Intel Core 2 CPUs it is not as true for the AMD CPUs. The difference is that the Core 2 processors have huge amounts of cache on them to eliminate/reduce the performance impacts of the extra latency introduced by Intel's FSB architecture. The AMD CPUs have less cache because they use an Integrated Memory Controller (IMC). The AMD CPUs connect directly to the memory bus and data moves directly from the DRAM onto the CPU chip. As a result the AMD CPUs use less cache and their performance tends to be more sensitive to changes in memory speeds and latencies.

(Sorry I can't be more precise than that. I don't know that much about AMD CPUs. All the reading I've done to date has pretty much been about Core 2. :) )

-john, the ostensibly clueless redundant legacy dinosaur
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